1. Field of the Invention
The present invention relates to a piezoelectric vibration piece, a piezoelectric vibrator, an oscillator, an electronic device and a radio-controlled timepiece.
2. Description of the Related Art
In recent years, a piezoelectric vibrator utilizing a crystal or the like has been used for a time source, a timing source for control signals, a reference signal source or the like of a mobile phone or portable information terminal device. Various piezoelectric vibrators of this type have been provided. As one of them, a piezoelectric vibrator having a tuning-fork type piezoelectric vibration piece is known.
By the way, the piezoelectric vibration piece needs to be smaller as the device in which the piezoelectric vibration piece is included becomes smaller. Generally, in order to make the piezoelectric vibration piece smaller while keeping low the CI (crystal impedance) value of the piezoelectric vibrator, a method is known in which a groove is formed in both main surfaces of the vibrating arms of the piezoelectric vibration piece. Furthermore, various proposals have been made for the shape of the groove of the vibrating arms.
For example, JP-A-2003-87090 proposes a vibration piece having: a base in which base electrodes (corresponding to mount electrodes of the invention) are formed; vibrating arms protruding from the base; grooves having groove electrodes (corresponding to excitation electrodes of the invention) formed on the front and/or back surfaces of the vibrating arms; side electrodes (corresponding to the excitation electrodes of the invention) formed on the side surfaces of the vibrating arms in which the grooves of the vibrating arms are not formed; and connection electrodes for side surface electrode (corresponding to leading electrodes of the invention) for connecting the base electrodes and the side electrodes, the opening of the grooves being formed such that the width of the base side is narrower than that of the other portion. Forming the width of the base side of the opening of the grooves narrower can ensure a large area in which the connection electrodes are formed. This is said to prevent the connection electrodes for groove electrode or connection electrodes for side surface electrode from short-circuiting with the groove electrodes or side surface electrodes to cause a defect in the vibration piece while preventing the CI value from increasing.
On the other hand, Japanese Patent No. 4,409,979 proposes that the width of the grooves near the base is gradually decreased in a tapered shape so that the width of the base side of the grooves is narrower than that of the other portion. Compared with the case of forming the grooves such that the width changes steeply as proposed in JP-A-2003-87090, in the case of forming the grooves in a tapered shape as proposed in Japanese Patent No. 4,409,979, etching residue is less likely to occur in the grooves. Thus, gradually decreasing the width of the grooves in a tapered shape allows accurate and stable etching of the grooves. This is said to reduce variation in shape of the left and right grooves to equalize the rigidity of legs and reduce the CI value.
Also, it is known that, when the piezoelectric vibration piece is activated, vibration of the piezoelectric vibration piece leaks to the outside through the base, i.e., what is called vibration leakage (vibration energy leakage) occurs. Since the vibration leakage may lead to increase in the CI value, the vibration leakage needs to be inhibited as much as possible.
One known method for inhibiting the vibration leakage is to form, in the base of the piezoelectric vibration piece, a mount section in which mount electrodes to be mounted outside are formed and an intermediate section located between the mount section and the vibrating arms, such that the mount section has a width wider than that of the intermediate section, and provide a step part to connect the mount section and the intermediate section. Where the characteristics of the vibration leakage depends on the length of the intermediate section in the longitudinal direction of the vibrating arms (hereinafter referred to as “effective value”).
The shape of the grooves and the effective value described above cause the CI value and the characteristics of the vibration leakage to vary, causing the vibration mode of the piezoelectric vibration piece to vary, which may influence the drive level characteristics of the piezoelectric vibration piece.
FIG. 15 illustrates the drive level characteristics.
The drive level characteristics refers to a variability characteristics of the vibration frequency with respect to the variation in drive voltage. Specifically, as shown in FIG. 15, when the voltage applied to the piezoelectric vibration piece is increased from V1 to V2, the frequency increases from f0 to f1. Then, when the voltage applied to the piezoelectric vibration piece is returned from V2 to V1, the frequency does not return from f1 to f0, but decreases to f2 lower than f0. The characteristics of this variation in the vibration frequency Δf (difference between f0 and f2) is referred to as the drive level characteristics. Note that, the smaller the amplitude of Δf is, the better the drive level characteristics is considered to be.
According to JP-A-2003-87090 and Japanese Patent No. 4,409,979, the grooves are formed such that the width of the base-end side is narrower than that of the tip side, which increases the rigidity of the base-end side of the vibrating arms that is largely distorted when vibrating. This causes the vibration mode of the piezoelectric vibration piece to vary while keeping the CI value low. Accordingly, this may also cause the drive level characteristics to vary.
By the way, the piezoelectric vibration piece is formed by etching a wafer of quartz. When the step part is provided to connect the mount section and the intermediate section, the amount of etching residue in a corner of the step part varies depending on etching time. Accordingly, the length of the intermediate section, i.e., the effective value varies depending on the etching time, which causes the characteristics of the vibration leakage to vary, causing the vibration mode of the piezoelectric vibration piece to vary. Accordingly, this may also cause the drive level characteristics to vary.
In this way, providing the step part to connect the mount section and the intermediate section causes the effective value to vary depending on the etching time, which limits the improvement of the drive level characteristics.